Generally used as isolators for high-frequency signals at present are three-port circulators whose one terminal is terminated by a matching impedance. Three-port circulators are classified into a distributed element circulator and a lumped element circulator. The circulator has a basic structure comprising a thin ferrite plate, a permanent magnet for applying a magnetic field to the thin ferrite plate perpendicularly, and conductors disposed around the thin ferrite plate, with irreversible electric characteristics. The distributed element is used when the size of the thin ferrite plate is ¼ or more of the wavelength of a high-frequency signal transmitting therethrough. The lumped element circulator is used when the size of the thin ferrite plate is ⅛ or less of the wavelength of a high-frequency signal transmitting therethrough. Accordingly, the lumped element circulator is more suitable for miniaturization than the distributed element circulator.
FIG. 7 is a schematic view showing an isolator circuit used for cell phones, etc. at present, which is constituted by connecting a matching impedance (resistor R) to one port of the three-port, lumped element circulator. Three central conductors L1, L2, L3 are disposed at an equal interval of 120° on the upper surface of a thin ferrite plate G composed of garnet-type ferrite. One end of each central conductor L1, L2, L3 serves as an input-output line for a terminal (1), (2), (3), and the other end is connected to a common terminal GR serving as a ground. Matching capacitors C1, C2, C3 are parallel-connected between the ends of the central conductors L1, L2, L3 and the common terminal GR. To operate as an isolator, an energy-absorbing resistor R is connected between the terminal (3) and the common terminal GR.
To apply a static magnetic field to the main surface of the thin ferrite plate G substantially in perpendicular thereto, a permanent magnet (not shown) is mounted onto a casing serving as a magnetic yoke. In the isolator shown in FIG. 7, at the desired frequency (hereinafter referred to as “center frequency”) f0, a high-frequency signal entering into the terminal (1) is transmitted to the terminal (2), and a high-frequency signal entering into the terminal (2) is transmitted to the terminal (3), respectively with small loss. However, because a resistor R is connected to the terminal (3), almost all energy is absorbed thereby, so that substantially no high-frequency signal is transmitted from the terminal (2) to the terminal (1). Thus, high-frequency signal is transmitted only in one direction in this isolator, with a high-frequency signal in the opposite direction prevented from transmission.
Though the isolator shown in FIG. 7 is advantageous in having small insertion loss in a wide bandwidth, it is disadvantageous in that its bandwidth in which large isolation loss is obtained is narrow. Because three central conductors cross at an angle of 120°, the coupling of the central conductors at a frequency quite higher than the desired frequency f0 cannot be neglected. A second peak of transmission loss thus appears in a high-frequency signal at about 1.4 f0 [S. Takeda; 1999 IEEE MTT-S Digest, pp. 1361-1364 (WEF 3-1)]. As a result, the isolation loss is degraded to about 5 dB. Under this influence, there is no large attenuation in a high-frequency signal in an opposite direction at 2f0 and 3f0.
On the other hand, the two-port isolator shown in FIG. 6 comprises two central conductors L1, L2 crossing perpendicularly. See, for instance, Japanese Patent Laid-Open No. 52-134349 (U.S. Pat. No. 4,016,510), and Japanese Patent Laid-Open No. 53-129561 (U.S. Pat. No. 4,101,850). Because of this structure, it is advantageous in that high attenuation in an opposite direction is obtained in a high-frequency even deviated from near a center frequency f0 called “within bandwidth”, at which a normal isolator operation is carried out.
In the two-port isolator having this structure, matching capacitors C1, C2 are connected in parallel between ends of the central conductors L1, L2 and the common terminal GR. An important feature of the two-port isolator is that two terminals of the energy-absorbing resistor R are connected to ends of the central conductors L1, L2. The other ends of the central conductors L1, L2 are connected to the common terminal GR as a ground. Because the two-port isolator is smaller than the three-port circulator by one central conductor and one matching capacitor, it is suitable for a small, thin isolator.
However, the two-port isolator having the structure shown in FIG. 6 has not been put into widespread practical use. The reason is that because the two-port isolator is disadvantageous in having a narrow bandwidth in which small insertion loss is obtained, though large isolation is obtained in a wide bandwidth, the insertion loss of the two-port isolator cannot be reduced to much smaller than that of the three-port circulator. One example of expanding the bandwidth may be to reduce a normalized operating magnetic field σ by making a static magnetic field applied to a thin ferrite plate smaller. However, this leads to an increase in insertion loss, because the ferrite has a large magnetic loss.
In addition, the operation principle of the two-port isolator has not been investigated in detail unlike the three-port circulator. Therefore, the inventions have developed a circuit simulator for analyzing the circuit of FIG. 6, and got a fundamental knowledge to large isolation loss and small insertion loss in a wide bandwidth based on the analysis results. The operation principle of FIG. 6 will be described below based on the circuit analysis.
When a high-frequency signal enters into the circuit through the terminal (1), electric current flows in the central conductor L1, thereby exciting the thin ferrite plate G. Because the thin ferrite plate G is magnetized in a direction of its main surface by the permanent magnet, a high-frequency magnetic field is generated from the thin ferrite plate G, exciting electric current in the central conductor L2 in perpendicular to the central conductor L1. This is due to the ferromagnetic resonant effect of ferrite in a microwave band. Because of this effect, the central conductor L1 is coupled to the central conductor L2, thereby enabling the transmission of a high-frequency energy from the central conductor L1 to the central conductor L2.
Respective pairs of the matching capacitors C1, C2 and the central conductors L1, L2 constitute parallel resonance circuits resonating at a center frequency f0. What should be paid attention is the change of phase when a high-frequency energy is transmitted. Namely, when energy is transmitted from the terminal (1) to the terminal (2), its phase difference is 0°, no electric current flows through the resistor R if the input and the output have the same amplitude. To the contrary, when energy is transmitted from the terminal (2) to the terminal (1), its phase difference is just 180°. In this case, large electric current flows through the energy-absorbing resistor R, resulting in the consumption of energy. Thus, energy is unlikely to be transmitted from the terminal (2) to the terminal (1).
FIGS. 3(a) and (b) show the insertion loss, isolation and reflection loss of such a conventional two-port isolator by the solid line. In the figure, a white triangle on the axis of ordinates indicates a reference line of 0 dB. As shown in FIG. 6, this two-port isolator has a structure in which a thin garnet plate G having a diameter of 3.9 mm and a thickness of 0.4 mm is disposed in a 7-mm-square iron casing having a ferrite magnet fixed to an inner surface thereof, two perpendicularly crossing central conductors L1, L2 are disposed in the vicinity of the ferrite magnet, and ceramic capacitors C1, C2 are added. The resistance of the resistor R is 50Ω. FIG. 3(a) shows the frequency characteristics of insertion loss and reflection loss of an input port (corresponding to the terminal (1) in FIG. 6), and FIG. 3(b) shows the frequency characteristics of isolation loss and reflection loss of an output port (corresponding to the terminal (2) in FIG. 6).
The minimum value (0.58 dB) of insertion loss occurs at a frequency of 1140 MHz (center frequency f0). This value is larger than the insertion loss of the three-port circulator by 0.2-0.3 dB. The isolation loss is about 11 dB at a center frequency f0, which is not necessarily so good. The frequency characteristics of the isolation loss of the two-port isolator are in an upward projecting curve, unlike a downward projecting curve in the three-port circulator.
FIG. 4 shows the insertion loss and isolation loss of the above two-port isolator measured in a wider frequency range than in FIG. 3. FIG. 4(a) shows the frequency characteristics of insertion loss and reflection loss of an input port, and FIG. 4(b) shows the frequency characteristics of isolation loss and reflection loss of an output port. FIGS. 4(a) (b) show attenuation at 2 f0, 3 f0, wherein f0 is a frequency of 1140 MHz at which the insertion loss is minimum. FIG. 4(b) also shows the insertion loss of FIG. 4(a) by a dotted line for comparison. As is clear from both figures, this isolator reflects almost all at frequencies of 2 f0 and 3 f0 outside the bandwidth, with the attenuation of transmission of about 30 dB. What is better is that there is no unnecessary resonance as seen in the three-port circulator. The insertion loss and isolation loss have upward curved frequency characteristics.
Another example of the two-port isolator has a structure in which two central conductors are sandwiched by two thin ferrite plate pieces. FIG. 8 shows the arrangement of central conductors L1, L2 and a thin ferrite plate G in such a two-port isolator. FIG. 8(a) is a plan view showing the arrangement of a first thin ferrite plate piece G1 and two central conductors L1, L2, with a second thin ferrite plate piece G2 omitted. FIG. 8(b) is a cross-sectional view taken along the line A—A in FIG. 8(a). The second central conductor L2 is perpendicularly disposed on the first central conductor L1 via an insulating layer. The second thin ferrite plate piece G2 is in close contact with the second central conductor L2. The arrow MF indicates a high-frequency magnetic field induced by a high-frequency electric current flowing through the central conductor L1.
Because a high-frequency magnetic field passes through a gap between the thin ferrite plate pieces G1, G2, the thin ferrite plate pieces G1, G2 cannot be excited efficiently because of a demagnetizing field in the gap. As a result, strong coupling cannot be obtained between the two central conductors L1, L2. It has been found by simulation that in a two-port isolator comprising central conductors L1, L2 crossing perpendicularly, the poor coupling of the central conductors L1, L2 leads to deterioration in insertion loss. When the second thin ferrite plate piece G2 is not used, coupling is further poor between the central conductors L1, L2. The solid lines in FIGS. 3(a) and (b) indicate the insertion loss, isolation loss and reflection loss of a two-port isolator comprising a thin ferrite plate consisting only of a first thin ferrite plate piece G1 without using a second thin ferrite plate piece G2.
FIG. 16(a) shows a combination of central conductors L1, L2 having two parallel conductor portions and a first, rectangular, thin ferrite plate G1 in the conventional two-port isolator, and FIG. 16(b) shows a second thin ferrite plate piece G2 disposed on the second central conductor L2 in close contact. The coupling of the central conductors L1, L2 is slightly larger in the assembly shown in FIG. 16 than in the assembly comprising a thin, circular ferrite plate as shown in FIG. 8.
The structure shown in FIG. 17 is the same as that shown in FIG. 16 except that two central conductors L1, L2 are knitted. Because of this structure, the coupling of the two central conductors L1, L2 can be improved.
It has been found by simulation that in a two-port isolator comprising central conductors L1, L2 crossing perpendicularly, the poor coupling of central conductors L1, L2 leads to deterioration in insertion loss. It has been found by analyzing the conventional structures shown in FIGS. 16 and 17 that two central conductors L1, L2 are not necessarily coupled efficiently throughout the rectangular, thin ferrite plate pieces G1, G2. Coupling was insufficient between the two central conductors particularly in the peripheral portions of the thin ferrite plates.
Practically, there is capacitance between the first and second central conductors, and there is parasitic inductance in series to a resistor. When such a parasitic element exists, the desired operation cannot be expected. It is thus desired to optimize by simulation the circuit characteristics of a two-port lumped element isolator. When the crossing angle φ of a center axis of the first central conductor L1 and a center axis of the second central conductor L2 is changed, simulation as to how these inter-conductor capacitance and parasitic inductance change is described in U.S. Pat. No. 4,210,886. However, its theoretical consideration is not clear, and the resultant crossing angle is not necessarily acceptable for practical purposes.
As described above, though the conventional two-port isolator provides large isolation loss in a wide bandwidth, it is disadvantageous in having large insertion loss at a center frequency f0 and a narrow bandwidth in which small insertion loss is obtained.